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espanolLa Sierra de Guasayan forma parte de las Sierras Pampeanas Orientales, en la provincia de Santiago del Estero, NW Argentino, y esta conformada por varios cuerpos graniticos que intruyen en un basamento metamorfico de bajo grado, el cual esta representado al norte de la sierra por pizarras y filitas, mientras que al sur predominan los esquistos bandeados con inyecciones cuarzo-feldespaticas. Sobre la base de sus caracteres sedimentarios, petrologicos y relaciones estratigraficas generales, las unidades metasedimentarias del sector norte se asocian al contexto estratigrafico de la cuenca de Puncoviscana. En la zona de contacto granito-metamorfitas se han reconocido corneanas cordieriticas desarrolladas por metamorfismo termico. Los granitoides constituyen un cinturon elongado de Norte a Sur, de composicion mayoritariamente tonalitica-granodioritica al norte y granitica-granodioritica al sur. Son predominantes las texturas porfiricas y tamano de grano medio. Geoquimicamente son rocas calcoalcalinas de caracter debilmente peraluminoso, con valores del indice de saturacion en alumina (ASI) entre 1 y 1.09. La uniformidad petrografica y geoquimica que presentan los cuerpos aflorantes indicaria una posible genesis en comun. Comparativamente, son similares a los granitoides cambricos presentes en el Batolito de Tastil, en el norte, en el ambito de la Cordillera Oriental y en parte a los del Batolito Sierra Norte-Ambargasta, que afloran hacia el sur. Todos ellos constituyen el Arco Magmatico Pampeano desarrollado en el margen occidental de Gondwana durante el Neoproterozoico tardio - Cambrico inferior EnglishThe Sierra de Guasayan is part of de Eastern Sierras Pampeanas, in the province of Santiago del Estero, NW Argentina, being mainly formed by several granite bodies emplaced in the regional low-grade metamorphic basement. The latter is represented by slates and phylites in the northern part of the range, whereas in the southern part, banded schists with quartz-feldspar veins are predominant. On the basis of their sedimentary, petrological and general stratigraphic relations, the metasedimentary units of the northern sector are related here to the Puncoviscana basin. Cordieritic hornfelds developed by thermal metamorphism have been recognized in the granite-metamorphites contact zone. The composition of the granitoids is mainly tonalitic-granodioritic to the north, and granitic-granodioritic to the south. Porphyritic textures and medium grained are predominant, while there is common occurrence of small rounded to elongated xenoliths. Geochemically, the granitoids are calc-alkaline, weakly peraluminous with ASI values between 1-1.09. Normal trends of evolution are show in the Harker diagrams. The petrographic and geochemical uniformity of the granitic rocks suggests a common genetic source. Comparatively, the granitoids of Sierra de Guasayan are similar to the Cambrian granitoids present to the north, in the Batolito de Tastil, Cordillera Oriental, and partly to those of the Batolito Sierra Norte-Ambargasta, to the south. All of them constitute the Pampean Magmatic Arc, developed on the western margin of Gondwana during the Late Neoproterozoic - Lower Cambrian

Mantle-derived volcanic rocks from the Subbetic hyperextended basin in SE Spain provide new insights into the composition and mechanical behavior of the mantle during continental rifting. The present study describes a sequential restored cross-section along with geochemical characteristics of the basaltic rocks interbedded within the Mesozoic succession of the basin. Sedimentary stacking patterns of minibasins above the mobilized salt reflect the relationships with coeval basaltic volcanism. We recognize two type localities on the basis of volcanic facies, the presence of shallow intrusive bodies, and age of the associated sedimentary formations. The first type corresponds to subaqueous pillow-lava flows and subvolcanic sills and dikes associated with Lower Jurassic marly limestones and Middle Jurassic oolitic limestones. The Jurassic basalts present enriched MORB compositions with moderate La/Sm and low Sm/Yb ratios. Interestingly, a significant group of this Jurassic basaltic magmatism departs from the typical MORB-OIB array, showing deep Nb-Ta negative anomalies and high Th/Nb ratios. The second type comprises subaqueous lava flows, also including pillow-shaped basalts interlayered with hyaloclastite deposits and Upper Cretaceous clays, radiolarites, and marly limestones. The Cretaceous magmatism is characterized by highly enriched MORB compositions. Furthermore, the moderate Sm/Yb values and the positive correlation between LREE/HREE and Zr point to the involvement of deep (Grt-present) mantle sources in the origin of the Cretaceous basaltic melts. We interpret the Lower- Middle Jurassic calc-alkaline signal as due to the partial melting of recycled crustal rocks within the upper mantle, i.e., associated with remnants of pre-Mesozoic subducted slabs. These characteristics are similar to those described in Triassic basaltic rocks widespread throughout the External Zone of the Betic Cordillera. Mantle-derived basalts interlayered within the Lower Jurassic syn-rift deposits indicate that melting and deformation within the lithospheric mantle was initiated early during continental rifting. Accordingly, we suggest that Early to Middle Jurassic mantle melts promoted failure within the upper mantle, thus contributing to the inception of lithospheric-scale shear zones, which, in turn, controlled the evolution of this magma-poor hyperextended margin. Subsequently, rift evolution gave way to the activation of deeper melt sources in the mantle and an increase of the alkaline signature at the Cretaceous time., Spanish Geological Survey, Junta de Andalucia A-RNM-005-UGR18 PY20-01387, Spanish Government REViSE-Betics-PID2020-119651RB-I00

The manuscript has been benefited from valuable suggestions from Mattia Pistone and Jiri Zak. This work is supported by IBERCRUST project (PGC2018-096534-B-I00). C. R. is grateful for her AUIP grants (Asociacion Universitaria Iberoamericana de Postgrado) for both stays at the Universidad Nacional de La Plata (Argentina), during 2015 and 2018, and for her postdoctoral contract from the University of Huelva (under the Estrategia Politica Cientifica de la UHU 2016/2017). This is the IBERSIMS publication n.85. JM is funded by the MINECO grant CGL2017-84901-C2-P1., The Paleocene-Eocene Cerro Munro pluton, to the east of the North Patagonian Batholith (NPB), is a tonalitic intrusion emplaced as a shallow, small, sub-circular tonalite-granodiorite body hosting abundant co-magmatic mafic microgranular enclaves (MME). Besides, the intrusive body is crosscut by radial porphyritic dikes and has been related to andesitic, dacitic and rhyolitic dikes, lava flows and ignimbrites cropping out at neighboring areas. Magmatic contacts between MME and host tonalites, together with their common geochemical features, suggest derivation from a common parental magma, although short-range mineralogical and geochemical differences point to an early crystallization of MME (chilled margins) at the sidewalls of ascent conduits or at shallow reservoirs. The established thermal gradient and the advance of the solidification front were responsible for the presence of the mafic microgranular and tonalitic subsystems and, in a continuous process, promoted the water saturation and the second boiling that finally account for the segregation of a water rich highly differentiated residual liquid. Hbl-Pl cumulate textures observed in the tonalites and mafic enclaves, as well as incompatible element-enriched rhyolitic melts record the results of this in-situ differentiation process. U–Pb zircon ages obtained from tonalites (57.1 ± 1.4 Ma), dacitic (55.4 ± 0.6 Ma) and rhyolitic (54.1 ± 0.7 Ma) dikes constrain an age of around 54 Ma for the final consolidation of the tonalitic magma and the crystallization of the expelled highly differentiated melts. This age coincides with the deposition of dacitic volcanic and volcanoclastic deposits to the north of the Cerro Munro pluton, which suggests that the fractionation process led to the extrusion of segregated melts, favored by extension, uplift and exhumation of an active rift tectonic setting., IBERCRUST project PGC2018-096534-B-I00, AUIP grants (Asociacion Universitaria Iberoamericana de Postgrado), University of Huelva, MINECO CGL2017-84901-C2-P1

The Chanaral tectonic melange (northern Chile) is a local unit within the late Paleozoic accretionary complex formed at the southwestern margin of Gondwana. The structural characteristics of the studied melange were mostly developed during a first deformation phase (D1) and include a block-in-matrix fabric, lineations (L1) and foliations (S1), tight to intrafoliar folds, S-C and S-C-C′ composite planar fabrics, and a conspicuous spatial separation of domains with predominantly linear and linear-planar fabrics. Folding during a second stage (D2) modified the orientation of the previous fabrics and structures. The eastern boundary of the Chanaral melange is N-S to NNW-SSE oriented, moderately dipping to the east. Its western boundary is not exposed. The plane showing the maximum structural asymmetry (the vorticity normal section) is ENE-WSW directed, and sub-vertical. Kinematic criteria consistently reveal top-to-the-WSW displacement. A kinematic model of triclinic transpression with inclined extrusion has been applied to evaluate the D1 structural features of the Chanaral melange. The pitch of the simple shear direction on the deformation plane ranged from 60°N to 90°. The pitch of the estimated extrusion direction was of 30°-40°S. The coaxial component was clearly constrictional (logarithmic K value of 2 to 5). The vorticity number has not been constrained by the model, but its spatial variation can explain the domainal distribution of the fabrics in the melange. The simulated particle paths show the predominance of material displacement parallel to the margin, with low to moderate down-dip displacements, which is in accordance with the low-pressure metamorphic assemblages found in the melange. The convergence direction between the blocks separated by the melange unit was N50°-60°E. Kinematic blocking of the Chanaral melange, probably related to the accretion of an oceanic volcanic domain, allowed the D2 folding of the previous structures, a process that, at least initially, proceeded without a change in the convergence direction.

Compositionally zoned plutons, both layered and concentrically arranged, provide granitic exposures where the mechanisms and timing of the magmatic emplacement processes can be studied. The importance of in-situ geochemical differentiation and the magma replenishment rates are revealed by geochemistry and field relations, together with the increasingly accurate U–Pb geochronology, which has promoted the knowledge about the pluton incremental assembly theories.The Flamenco pluton, located in the Coastal Range of northern Chile, is part of the Upper Triassic to Early Cretaceous Andean intrusives formed in the western active margin of South America, and present a normal zoned structure with mafic magmatic facies (mostly gabbros and Qtz-diorites) close to the contacts with the host metasediments, and tonalites, granodiorites and granites in the inner areas. A combined study of the field relations, geochemistry and zircon geochronology of the magmatic facies was applied to determine the emplacement sequence of the Flamenco pluton, revealing three distinguishable domains separated by metasedimentary septa. The SW area is constituted by mostly homogeneous leucocratic granodiorites that yielded an age of 213 Ma as the best estimation for their emplacement age. Distinctive geochemical characteristics, such as the absence of an Eu anomaly, the depletion in HREE, or the highest Sr, Sr/Y and Ce/Yb values among the granodioritic facies of the pluton, involve lower T and/or higher P conditions at the magmatic source according to experimental studies. These conditions were established during an early stage of the Andean magmatic arc building that is firstly defined here as Upper Triassic. The NW and E domains of the pluton were sequentially emplaced between 194 Ma and 186 Ma and both the field relations and the detailed geochronological results suggest that the mafic facies intruded latter in the emplacement sequence. To the NW, Qtz-dioritic and gabbroic externally emplaced pulses gave a younger crystallization age of 186.3 ± 1.8 Ma, and promoted the granoblastic textures and metamorphic zircon overgrowths that characterize the granodiorites located in the contact with the intermediate and felsic inner magmas, which yielded a best estimation of their emplacement age of 192 ± 1.5 Ma. On the other hand, in the eastern domain, magma-magma relations are observed between gabbros and previously intruded tonalites and granodiorites. Both the mafic and intermediate facies show two main subgroups of ages that yielded 194.7 ± 1.5 Ma to 188.3 ± 2.1 Ma and 193.1 ± 2.2 Ma to 185.5 ± 1.4 Ma respectively. These differences are related to the variations in the magmatic addition rates, which may extend the super-solidus conditions in the eastern domain of the magmatic reservoir as is confirmed by the wider age ranges yielded by these magmatic facies. Zircon overgrowths in the host rocks yield similar ages (around 220 Ma and 205 Ma) than the oldest results obtained in the intrusive facies, indicating that metamorphism correlates with the initial stages of plutonic emplacement.Geochronological results differ between 9 Myr and 41 Myr in the eight studied samples for non-inherited ages and gave very close mean ages (within analytical uncertainty) for all the intrusive units. However, we examine other characteristics such as zircon morphology, internal structure, geochemistry and statistical data to assess if the scattering of the geochronological data may be related to the different processes involved in the construction of the Flamenco pluton. We concluded that this detailed study of U–Pb zircon ages, including individual and significative groups of analyses, is useful to determine accurately the emplacement sequence and the genetic relation between the intrusive units, together with the evidences depicted by the geochemistry and field relations. Keywords: Zircon U–Pb geochronology, Zoned plutons, Andean magmatic arc, Sequential emplacement, Individual and statistical zircon ages

Earth and Space Science Open Archive., Differentiation and contamination of silicic magmas are common phenomena characterizing the granite batholiths and large igneous provinces that build up most of the continental crust. Although they can be identified by means of geochemical relations of igneous rocks exposed in the continents, the mechanisms allowing magmas to undergo the necessary crystal–liquid separation and digestion of country rocks for differentiation and contamination are poorly constrained. In this paper we show two independent approaches that are essential to understand fractionation and contamination of magmas. These are (1) the study and interpretation of field relations in exposed deep sections of batholiths, and (2) the results of laboratory experiments carried out at middle–upper crust pressure. Experiments support that fractionation is intrinsic to crystallization of water-bearing magmas in thermal boundary layers created at the sidewalls of ascent conduits and walls of magma chambers. Gravitational collapse and fluid migration are processes identified in experimental capsules. Similarly, reaction experiments in mixed capsules support reactive bulk assimilation as a plausible mechanism that is compatible with field and petrographic observations in contaminated granitic rocks., This work is supported by the State Agency for Research (AEI) (Project: PGC2018– 096534–B–I00). C.R. is grateful for her postdoctoral contract from the University of Huelva (Grant: Estrategia de Politíca Científica UHU 2016/2017).

The Las Tortolas Formation extends from 29° to 26° S along the northern chilean coast and has been interpreted as a Carboniferous accretionary prism. This essentially metasedimentary turbiditic formation constituted the late Paleozoic basement of the Upper Triassic-Jurassic magmatism during the inception of the Andean cycle in northern Chile. The geochemical characteristics and field relations studied in the volcanic rocks of the Las Tortolas Formation, described with detail in this work, mean a key fact to assess the notable paleogeographic changes occurred during the pre-Andean cycle in the western margin of South America. The volcanic domain included in the Las Tortolas Formation is mainly comprised by basaltic rocks, with minor tuffs and rhyolites, and presents thrust-fault contacts with the melange facies to the west and the metasedimentary Eastern Series to the east. Together, the Las Tortolas Formation was conformed by the stacking of the basal highly deformed units of this accretionary complex, represented by the melange facies, and the volcanic domain that became overlain by the turbiditic sequences at their top. The geochemical characteristics of the Las Tortolas basaltic rocks involve an enriched mantle source, as a mantle plume. In spite of the N-MORB signature observed in the basal hyaloclastitic basalts, the lava flows, as the metabasites sampled in the melange facies, yield E-MORB characteristics according to the multi-element and tectonic discrimination diagrams and several immobile element ratios as the Th/Yb, TiO2/Yb and Nb/Yb, among others. Therefore, the origin of the volcanic domain of the Las Tortolas Formation may be related to a hot mantle plume impinged on the base of the oceanic lithosphere under the tip of the overriding plate, which firstly promoted the partial melting of the depleted surrounding lithosphere (N-MORB hyaloclastites) and continued with the extrusion of lava flows and pyroclastic rocks derived from the enriched source, interlayered with the distal turbiditic succession. The development of the magmatic system along the oceanic crust enabled the formation of magma chambers where small volumes of rhyolitic melts were fractionated. The geochemical evolution of this basaltic magmatism was correlated with the building of volcanic edifices during a first submarine stage (hyaloclastites, pillows and lava flows) and a subsequent sub-aerial explosive phase depicted by sedimentary and rhyolitic breccias. Finally, we explore a tectonic setting defined by a plume overridden by the lithosphere during the relative convergence with the continental margin to explain the formation of the volcanic domain and its accretion to the sedimentary units. Besides, this tectonic configuration may be taken into account as the driving force of the tectonic and magmatic events that characterize the pre-Andean cycle.

Trace elements, isotopic modeling and U-Th-Pb SHRIMP zircon age constraints are used to reconstruct the eruption history and magmatic processes of the Piedra Parada Caldera. In the early Eocene, the crystal-poor Barda Colorada ignimbrite (BCI), having >>15% micro-porphyritic crystals with respect to magmatic components, erupted a volume estimated in more than 300 km3. The Piedra Parada caldera is located in the Patagonian Andes foreland, at the southern end of the calderas field of the Pilcaniyeu Volcanic Belt (PVB). This belt is related to an extensional tectonic setting as a result of the collision of the Farallon-Aluk ridge with South America, which enabled the development of a transform ocean/continental plate margin followed by the detachment of the Aluk plate and the opening of a slab window. The BCI extra-caldera Plateau is a >100 m thick deposit, having a lower unit with high silica (SiO2 > 76 wt.%), potassium poor rhyolitic composition (trondhjemitic like magma), and an upper unit with normal to high potassium rhyolitic composition (granitic like magma). A trace elements modeling of the BCI units shows that the BCI lower and upper units did not evolve from fractionation or immiscibility in the shallow magma reservoir. The BCI also have a primitive isotopic signature (initial 87Sr/86Sr = 0.7031-0.7049 and ε Nd = +3.4 to +3.65). Thus, tectonic, compositional and isotopic constraints suggest the fast ascent of high silica magmas to a shallow reservoir, and point to an upper mantle origin for these rhyolitic magmas in a transitional (Orogenic-Anorogenic) tectono-magmatic setting. U-Th-Pb SHRIMP zircon crystallization ages of the Syn-caldera stage BCI units (56-51.5 Ma) show a protracted life of 5 Ma for this caldera reservoir. The age of 52.9 ± 0.3 Ma is considered the best fit for the possible maximum age for the caldera collapse. The Late-caldera magmatism has trachyandesitic and rhyolitic compositions. The trace element modeling suggests that these rhyolites evolve from the trachyandesites and do not evolve from the BCI residual magma. The trachyandesites have U-Th-Pb SHRIMP zircon crystallization ages of 52 ± 1 Ma, suggesting that the caldera eruption was triggered by the arrival of the trachyandesitic magma. Fil: Aragon, Eugenio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina Fil: Castro, Antonio. Universidad de Huelva; España Fil: Diaz Alvarado, Juan. Universidad de Atacama; Chile Fil: Pinotti, Lucio Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; Argentina Fil: D'eramo, Fernando Javier. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; Argentina Fil: Demartis, Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; Argentina Fil: Coniglio, Jorge Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas, Fisicoquímicas y Naturales. Departamento de Geología; Argentina Fil: Hernando, Irene Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; Argentina Fil: Rodriguez, Carmen. Universidad de Huelva; España

Oblique convergent margins, like the subduction of Phoenix beneath the South American plate during Jurassic and Early Cretaceous times, are characterized by strain partitioning and a positive feedback loop between strike-slip deformation and magma ascent along the magmatic arc. Located in the Coastal batholith of northern Chile, the Flamenco pluton is one of the youngest Andean intrusives emplaced in the western active margin of South America. Besides an older SW domain of granodioritic rocks (c.a. 213 Ma), the NW and E domains of the Flamenco pluton were emplaced between 194 and 186 Ma. They present a normally zoned structure constituted by external gabbroic to Qtz-dioritic magmatic facies and tonalites and granodiorites located in inner areas of the intrusive body. These domains are separated by a central strip of stretched coarse-grained Crd-schists that presents ductile asymmetrical folding and S–C structures that point to the NW-directed displacement of the E domain of the pluton. This syn-emplacement shear zone shows kinematic compatibility and continuity to the north and south with folded and mylonitic metasediments out of the contact aureole of the pluton. Together, these segments constitute a large, steeply dipping sigmoidal structure of average N–S direction; the called here Chanaral transcurrent shear zone. Contemporary to the emplacement of the Flamenco pluton, slight variations in the trend of the crustal-scale structure generated strike-slip and transpressive sectors along the Chanaral shear zone, which favored the access of intruding magmas to the final emplacement level. As a paradigmatic example, the curviplanar Flamenco shear zone, an internal, magmatic branch of the main structure that traverses the E domain of the pluton, is defined by the sinistral and reverse shearing under magmatic conditions of the previously mingled mafic and felsic batches. Consequently, the transpressive Flamenco shear zone is interpreted as an ascent conduit where gabbroic and granodioritic liquids interacted during the building of the intrusive body. In addition, these sheared rocks were affected by late textural coarsening processes that evidence the slow and cyclical cooling of the growing magma reservoir. In contrast with the steeply dipping contacts and structures found to the east, the NW domain of the pluton shows sharp and gently dipping contacts between almost horizontal magmatic layers. We suggest that the western block of the Chanaral shear zone was a relatively passive footwall dominated by horizontal flow trajectories and lower replenishment rates according to the inverse emplacement sequence, i.e., late external mafic batches intruded along the margins of the felsic core. The variable structural arrangement of the crustal rocks that hosted the Flamenco pluton was the result of the complex interaction between far-field and local, magmatic forces during the emplacement process, besides the interference with pre-Andean structures. The presence of the Chanaral shear zone favoring the emplacement of the Flamenco pluton demonstrates that the Late Jurassic to latest Early Cretaceous Atacama Fault System had earlier precursors and both the magmatic arc axis and the transcurrent shear zones migrated landward during Jurassic times.

The Cerro Munro sub-volcanic intrusion is emplaced in the back-arc (400 km from the trench) as small sub-circular tonalite-granodiorite plutons with abundant radial porphyritic dikes. U-Pb zircon SHRIMP data give an age of crystallization of 57 Ma ± 1.4 Ma. It is located to the east of the North Patagonian Batholith (NPB) that shows a protracted and episodic magmatic history from Cretaceous to Miocene time. The NPB Palaeogene episode is characterized by the lack of magmatic activity at the arc axis, as small plutonic emplacements move to the fore-arc and back-arc. This Palaeogene tectono-magmatic episode is ruled by the detachment of the Aluk plate during the Aluk-Farallon-SAM triple junction, active at that time along northern Patagonia active margin, changing the Cretaceous ?NPB orogenic? setting to a Palaeogene ?Munro transitional? tectono-magmatic setting. Near the contacts, the tonalite contains abundant enclaves of igneous appearance and variable size from several cm to dm, described as autoliths. The study of autoliths and host tonalite reveals interesting results on the processes of fractionation in a thermally zoned magma chamber. Autoliths, and in a large extent the host tonalite, represent disguised cumulates from which a hydrous silicic liquid was extracted. Barometry calculations from mineral chemistry in both autoliths and tonalites record a shallow pressure of emplacement of 0.5 kbar. Rhyolite-dacite flows and ignimbrites, surrounding the northern contact of the Cerro Munro tonalite, may represent the exsolved liquid from the plutonic cumulates. The study by cathodoluminiscence and electron backscattered diffraction techniques from a rhyolite-hosted quartz supports this protracted history of the Cerro Munro magma chamber., Facultad de Ciencias Naturales y Museo, Centro de Investigaciones Geológicas, Consejo Nacional de Investigaciones Científicas y Técnicas

The orbicular granitoid of Caldera, located at the northern part of the Chilean Coastal Range, is a spectacular example of radial textures in orbicular structures. The orbicular body crops out as a 375 m 2 tabular to lensoidal intrusive sheet emplaced in the Lower Jurassic Relincho pluton. The orbicular structures are 3–7 cm in diameter ellipsoids hosted in a porphyritic matrix. The orbicules are comprised by a Qtz-dioritic core (3–5 cm in diameter) composed by Pl + Hbl + Qtz + Bt ± Kfs with equiaxial textures and a gabbroic shell (2–3 cm in diameter) characterized by feathery and radiate textures with a plagioclase + hornblende paragenesis. The radial shell crystals are rooted and orthogonally disposed in the irregular contact with the core. The radial shell, called here inner shell, is in contact with the granodioritic equiaxial interorbicular matrix through a 2–3 mm wide poikilitic band around the orbicule (outer shell). The outer shell and the matrix surrounding the orbicules are characterized by the presence of large hornblende and biotite oikocrystals that include fine-grained rounded plagioclase and magnetite. The oikocrystals of both the outer shell and the matrix have a circumferential arrangement around the orbicule, i.e. orthogonal to the radial inner shell. The coarse-grained granodioritic interorbicular matrix present pegmatitic domains with large acicular hornblende and K-feldspar megacrysts. This work presents a review of the textural characteristics of the orbicules and a complete new mineral and whole-rock geochemical study of the different parts of the orbicular granitoid, together with thermobarometric and crystallographic data, and theoretical modeling of the crystallization and element partitioning processes. We propose a model for the formation of the orbicular radial textures consisting of several processes that are suggested to occur fast and consecutively: superheating, volatile exsolution, undercooling, geochemical fractionation and columnar and equiaxial crystallization. According to the obtained results, the formation of the orbicular granitoid of Caldera may have initiated 1) during the generation of a magmatic fracture in the crystallization front of the Relincho pluton, where the water released by the host crystal mush was dissolved in the new batch of dioritic magma. 2) The high influx of water-rich liquids induced superheating conditions in the newly intruding magma that became a depolymerized liquid, where the only solid particules were the small irregular fragments of the host mush dragged from the fracture walls. 3) Volatile exsolution promoted crystallization under undercooling conditions. 4) Undercooling and nucleation around the core (cold germs) involved the physical and geochemical fractionation between two sub-systems: a gabbroic sub-system that comprises the solid paragenesis with a residual water-rich liquid and a granodioritic sub-system. 5) The orbicules, including core and inner shell, behaved as viscous bodies (crystals + residual liquid) floating in the granodioritic magma. 6) Higher undercooling rates occurred at the starting stage, close to the liquidus, promoting columnar crystallization around the cores and formation of the shells. Conversely, in the granodioritic matrix sub-system, equiaxial crystallization was promoted by low relative crystallization rates. 7) The rest of the crystallization process evolved later in the outer shell and the matrix, as suggested by the poikilitic textures observed in both sides of the orbicule contact, and under conditions close to the solidus of both sub-systems (shell and matrix). The water-rich residual liquid expelled during the orbicular shell crystallization was mingled with the partially crystallized matrix magma, generating the pegmatitic domains with large Kfs megacrysts.

Plinian-type eruptions are extremely hazardous, producing pyroclastic fallout and flows extending many kilometres from the vent. The most commonly invoked eruption trigger for Plinian-type eruptions is the intrusion of fresh magma, generally associated with precursory ground deformation and seismicity days/weeks before eruption. Closed-system internal triggering has also been proposed, such as protracted crystallisation of magma, which can produce a build-up of exsolved volatiles and thus pressurise the system prior to eruption. On 22–23 April 2015 Calbuco volcano, Chile, produced a sub-Plinian eruption with 2O) and formed phenocrysts of titanomagnetite, orthopyroxene, clinopyroxene and plagioclase cores (An78–93). Gradual cooling of the magma chamber produced thermal gradients and magma convection, evidenced by plagioclase overgrowth rims (An58–77) and blocky microlites (25–250 μm). Our interpretation is that this continuing crystallisation induced second boiling and an over-pressurisation of the system, leading to the rapid onset of the 2015 eruption. Petrological and geochemical evidence therefore shows that a closed-system magma chamber can evolve into a highly explosive eruption with very little precursory warning, posing a challenge for current volcano monitoring paradigms. We propose that internal triggering should be carefully considered as a mechanism for unexpected sub-Plinian eruptions, prompting a potential revision of existing hazard management strategies.

The metamorphic formations that outcrop along the northern and central Chilean coast (26°–39°S) represent the late Paleozoic accretionary complex at the convergent southwestern margin of Gondwana. The characteristics of the HP-LT basal units (Western Series) and the low metamorphic-grade Eastern Series depict a homogeneous margin where the basal and frontal accretionary processes were active between the late Mississippian and the early-middle Permian, respectively. The subsequent stage (c.270–210 Ma) was characterized by the tectonic and magmatism differences along a segmented margin. These changes may be triggered by the basal and frontal accretion of E- to N-MORB oceanic basalts that are present in the accretionary complexes. Both the inland location of the accretionary complexes during the transition to the Andean cycle (Late Triassic), as well as the presence of these voluminous and extended basaltic accreted formations, are consistent with the accretion of oceanic reliefs as a key factor to decipher the early Andean tectonic evolution.

The Bay of Biscay-Pyrenean rift system is a world-class example of an inverted hyperextended rift. However, tectonic questions such as the linkage of the extensional system, the magnitude of crustal extension, the interpretation of the syn-rift sequences and the amount of shortening during tectonic inversion remain controversial. This paper presents a sequentially restored crustal-scale transect across a key sector of the central Basque-Cantabrian Basin, which allows the prevailing tectonic models to be evaluated. Two separated phases of rifting during Permo-Triassic and late Jurassic-Early Cretaceous times led to breakup. In the second phase, a southerly-dipping extensional detachment accommodated at least 28 km of horizontal extension while separating the Iberian and European plates, and exhuming mantle rocks in the footwall. The locus of volcanism migrated to the north, peaking during the hyperextension and post-rift stages. Mantle sources were influenced by deep OIB-type components most likely triggered by the ascending asthenospheric mantle. Therefore, gravitational forces linked to asthenospheric doming are interpreted as the primary driving mechanism during the peak strain rate. Evacuation of Upper Triassic salt during the syn-rift and post-rift stages determined the development of a salt-floored ramp-syncline basin next to the lower detachment ramp, and the sinking of minibasins in other areas. About 41 km of Cenozoic shortening (21%) were resolved by underthrusting of the crust beneath the shallow lithospheric mantle, leading to the formation of thrusts with opposite vergences at the distal basin boundaries, and by the reactivation of the extensional detachment as a thrust fault. The buttress of the overburden over the salt in the hyperextended domain created outstanding examples of squeezed diapirs, chevron folds and thrust splays. Our results have implications for a better understanding of the connection between the Atlantic Ocean and the Western Tethys along the Bay of Biscay-Pyrenean realm, the processes of crust denudation prior to breakup, and the incorporation of rifted domains into collisional orogens.

U–Pb geochronological study of zircons from nodular granites and Qtz-diorites comprising part of Variscan high-grade metamorphic complexes in Gredos massif (Spanish Central System batholith) points out the significant presence of Cambro-Ordovician protoliths among the Variscan migmatitic rocks that host the Late Carboniferous intrusive granitoids. Indeed, the studied zone was affected by two contrasted tectono-magmatic episodes, Carboniferous (Variscan) and Cambro-Ordovician. Three main characteristics denote a close relation between the Cambro-Ordovician protholiths of the Prado de las Pozas high-grade metamorphic complex, strongly reworked during the Variscan Orogeny, and other Cambro-Ordovician igneous domains in the Central Iberian Zone of the Iberian Massif: (1) geochemical features show the ferrosilicic signature of nodular granites. They plot very close to the average analysis of the metavolcanic rocks of the Ollo de Sapo formation (Iberia). Qtz-diorites present typical calc-alkaline signatures and are geochemically similar to intermediate cordilleran granitoids. (2) Both Qtz-diorite and nodular granite samples yield a significant population of Cambro-Ordovician ages, ranging between 483 and 473 Ma and between 487 and 457 Ma, respectively. Besides, (3) the abundance of zircon inheritance observed on nodular granites matches the significant component of inheritance reported on Cambro-Ordovician metagranites and metavolcanic rocks of central and NW Iberia. The spatial and temporal coincidence of both peraluminous and intermediate granitoids, and specifically in nodular granites and Qtz-diorite enclaves of the Prado de las Pozas high-grade complex, is conducive to a common petrogenetic context for the formation of both magmatic types. Tectonic and geochemical characteristics describe the activity of a Cambro-Ordovician arc-back-arc tectonic setting associated with the subduction of the Iapetus–Tornquist Ocean and the birth of the Rheic Ocean. The extensional setting is favorable for the generation, emplacement, and fast rise of subduction-related cold diapirs, supported by the presence of typical calc-alkaline cordilleran granitoids contemporary with ferrosilicic volcanism., Part of this work was developed during the PhD Thesis of J.D.A., carried out at the Departments of Geology and Geodynamics and Paleontology, University of Huelva, with a Grant Fellowship from the Spanish Ministry of Science and Innovation (grant no. AP2005-3498). The work was funded with projects CGL2004-06808-C04-01/BTE, CGL2004-06808-C04-02/BTE, CGL2007-63237/BTE, and CGL2010-22022-C02-01 of the Spanish Ministry of Science and Innovation, and supported by project DIUDA-22268 of the University of Atacama. This work pays tribute to and relies on the tireless activity of Cecilio Quesada to help to unravel the intricacies of the Variscan and pre-Variscan tectonic evolution of Iberia. Richard Armstrong assisted for the technical support with SHRIMP work at ANU (Canberra) and Xie Hangqiang at SHRIMP Beijing Center (China). We would like to sincerely thank the editor and the reviewers (Ruben Diez Fernandez and others) of Tectonophysics for their thorough corrections of the first draft of this manuscript.

Along more than 1000 km, the boundary between the Pampean and Famatinian belts separates the Cambrian magmatism to the east (540-515 Ma) from the easternmost granitic rocks ascribed to the Famatinian orogeny to the west (early Ordovician). The geochronological and geochemical study presented in this work has revealed the first Cambrian magmatism in the Sierras de Aconquija and Cumbres Calchaquies at the northeastern edge of the Famatinian belt. Pabellon (522 ± 2.8 Ma) and Nunorco Grande (478.3 ± 2.3 Ma) intrusive units form part of the small plutons exposed together in the Tafi del Valle area, which have been related according to their geochemical characteristics. The good correlation observed between ages and characteristic geochemical signatures have motivated the comparison with other intrusive complexes along the Pampean and eastern Famatinian belts, resulting in regional diagnostic geochemical features and a petrogenetic and evolutive proposal for the Cambrian and early Ordovician magmatism in the eastern Sierras Pampeanas. Easternmost early Ordovician granitic rocks constitute a calcic, weak to moderately peraluminous series. They are Sr-rich tonalitic to monzogranitic magmas (SiO2 >62 %wt.) with steep REE patterns, positive and negative Eu anomalies, high Sr/Y and moderate Ce/Yb ratios. Accordingly, the water-fluxed melting of a mafic source under high to moderate pressures may be involved in the origin of silica- and Ca-rich parental magma. The shallower evolution of these melts consists of Pl-dominated fractionation trends that finally constituted Pl-rich crystal mushes (tonalites) and more felsic residual liquids. Regarding Cambrian granodiorites and monzogranites, any petrogenetic scheme must consider the magnesian, high-K calc-alkalic characteristics for a short-range silica variation, low to moderate peraluminosity and high zircon saturation temperatures (>850 °C). Source conditions are determined by the presence of Pl and Hbl in the solid assemblage based on the REE patterns. These geochemical characteristics point to the extensive HT-LP melting of a dominantly crustal source, probably assisted by the injection of hot H2O-rich mafic melts.

In the Precordillera of the Atacama region (~27°S), the Puquios chaos corresponds to a singular tectonic unit traditionally ascribed to a mid-Cretaceous extensional stage affecting the Mesozoic formations that were deposited between the arc and the backarc settings of the Andean margin. The melange-like unit consists of blocks of limestones in a mainly volcanic matrix that includes andesitic lavas, volcanic breccias and red vulcarenites of Upper Jurassic to Lower Cretaceous age. These rocks constitute a conformable succession of andesites, limestones and vulcarenites (from the base to the top) in the Cerro Aguila strata, observed to the south of the study area. The Puquios chaos is characterized by often isolated, tight to isoclinal folds and a set of low- and high-angle normal faults that were responsible for the block-in-matrix mesoscopic structure of the tectonic unit. These structures are easily observable in the calcareous rocks as the volcanic matrix shows a tectonically brecciated texture. The detailed structural study presented in this work allows us to propose the deformational mechanisms that finally constituted the apparently chaotic unit, including the restoration of the structures related to late contractive stages. Thus, we suggest that the singularity of the Puquios chaos is associated with a local, wide, extensional shear zone linked to a regional detachment surface. The intimate relation between recumbent folds and low- and high-angle normal faults, besides the similar orientation of fold hinges and fault planes, point to the actuation of a brittle-ductile shear zone where the calcareous competent layers gave rise to decametric-scale folds as the volcanic matrix responded to strain by brecciation. Stretching calculations carried out in a domino fault system yielded horizontal extension of around 30% for the high-angle faults. We obtained a relative age of the Puquios chaos formation according to the field relations and the structural determinations. A NE-directed intense extensional stage promoted a significant crustal thinning that exhumed but not affected the late Paleozoic basement. The Puquios structures and the related regional detachments affected the pre-Barremian arc and back-arc formations and were unconformably covered by the latest Cretaceous and Paleocene thick volcanic and volcanosedimentary deposits.

The ongoing eruptive cycle of Tungurahua volcano (Ecuador) since 1999 has been characterized by over 15 paroxysmal phases interrupted by periods of relative calm. Those phases included Strombolians, Vulcanians and one Subplinian eruptions and they generated tephra fallouts, pyroclastic density currents (PDCs) and lava flows. The 01 February 2014 eruption occurred after 75 days of quiescence. Two days before the eruption, a gradual increase of seismicity associated with sporadic weak ash emissions occured. Between 13:30 and 16:30 UTC of the 01 February, a swarm of volcano tectonic and long period earthquakes was detected and announced the possibility of an eruption within hours or days. After a few hours without surface manifestations, two short-lived Vulcanian explosions triggered the paroxysmal phase at 22:39 UTC which lasted 40 min and produced an eruptive column 13.4 km in height sustained during about 9 min. The activity evolved at 23:36 UTC into sporadic Strombolian explosions with discrete ash emissions and continued for several weeks. Both tephra fall and PDCs were studied for their dispersal, sedimentology, volume and eruption source parameters. Tephra was distributed around 240° to the S-SW of the volcano, and the bulk deposit volume is estimated to be 1.53 ± 0.35 × 10−2 km3 (4.76 ± 2.23 × 106 m3 DRE; VEI 3). PDCs descended by 9 ravines of the N-NW flanks. It was one of Tungurahua’s largest eruptions, after the August 2006 Subplinian event. The Vulcanian eruptive mechanism is interpreted to be related to a steady magma ascent and the rise in over-pressure in a blocked conduit (plug) and/or a depressurized solidification front. The transition to Strombolian style is well documented from the tephra fall componentry. In any of the interpretative scenari, the short-lived precursors for such a major event as well as the unusual tephra dispersion pattern urge for renewed hazard considerations at Tungurahua.

The Tungurahua volcano (Northern Andean Volcanic Zone) has been erupting since 1999, with at least four eruptive phases up to present. Although a dozen of research focuses in tephra fall deposits during this period, none of them cover the full eruptive cycle. We investigated the eruptive mechanisms and tephra fall deposition processes at Tungurahua between 1999 and 2014, through systematic analyses of tephra samples collected westward of the volcano using mechanical sieving grain size analysis, lithology, scanning electron microscopy, X-Ray fluorescence and X-Ray diffraction. Tephra is compounded by varying amounts of scoria (black and brown), lithics, hydrothermally altered fragments, pumice, glass shards and free crystals. Textural analyses of juvenile grains (scoria, pumice and glass shards) revealed a diversity of features concerning to their vesicularity, shape and surface/perimeter. Initially, tephra was characterized by hydrothermally altered fragments related to a phreatic phase which then evolved to a pure magmatic activity with Strombolian eruptions. A homogeneous andesitic composition was observed between 1999 and 2003; however silicarich compositions occurred later in 2006. Similarly, the mineral assemblage contained plagioclase, pyroxene and olivine, but magnetite and akermanite were then included during 2006, thus indicating the eruption of a new, probably mixed magma. As consequence, Plinian activity occurred in August 2006. Further activity in 2007 ejected notable amounts (40-65%) of recycled material during Vulcanian eruptions. New eruptions occurred between 2008 and 2010, and juvenile ash revealed the interplay between brittle and ductile fragmentation through ash explosions, jetting events and Strombolian activity. The activity between 2010 and 2012 incorporated hydrothermally altered material at time that eruptive silences became longer and frequent, thus suggesting the development of a sporadic hydrothermal system. Finally, between 2013 and 2014 a series of Vulcanian events occurred. Observed grain size distributions allow us to propose three different processes occurring during tephra deposition: 1) deposition of multiple ash plumes, 2) contributions from elutriated pyroclastic density currents or grain size mixing due to major eruptions, and 3) the aggregation of particles due to rain and/or lighting. From mineralogy and grain size we infer that exposition to ash may produce acute human health effects.

The study of a magmatic series composed of Qtz-diorites to granites of the south part of the Sierra de Velasco batholith (Famatinian arc) in Argentina reveals that most rocks follow a coherent cotectic trend, which is comparable to a cotectic line of liquids (CLL) traced from experimental liquid compositions of calc-alkaline (andesitic) systems. The identification of cotectic relationships is crucial to interpret the meaning of geochemical variation trends in terms of phase equilibria. We show the contrast between typical CLL trends defined by rocks of the Palanche pluton and rocks departing from CLL array, which are interpreted as resulting from local processes of bulk assimilation of a pelitic host. Zircon U–Pb age determinations with SHRIMP support a coeval zircon crystallization record from of about 480 to 450 Ma of samples plotting on the CLL. The absence of intrusive contacts between these coeval cotectic granites implies that they were fractionated in situ at the level of emplacement. Other younger pulses (442 ± 5 Ma), also fall on the CLL, denoting that uniform processes of melting and fractionation were operative for at least 4 0 Ma at the same locus at the active continental margin of Gondwana. During this protracted magmatic activity, sporadic pulses of mafic magma were injected (e.g., at 456 ± 7 Ma), revealing a complex process of pulse amalgamation in the building up of the batholith. These mafic magma pulses may have contributed to basification of the host granitoid by enclave dissolution at the low pressure of emplacement. The reported geochemical trends, as true CLL, together with observed textures and crystallization sequences, point to a parental intermediate system of andesite composition (SiO 2 > 55 wt.%) with a water content of about 2.5 wt.% H 2 O, similar to those formed by sediment–basalt subducted melanges. A sublithospheric origin for the mafic precursor is proposed in agreement with prediction by recently published thermo-mechanical modeling and experimental phase equilibria.

Large granite batholiths were emplaced at the Gondwana active margin during Lower Ordovician in South America. These have contributed to crustal growth by net addition of silicic rocks to the continental crust. New U–Pb SHRIMP zircon age determinations, together with thermobarometric and geochemical data, yield that batholith magma intrusion is the responsible of heating and self-granulitization of early gabbro pulses. Partially molten granulitic gabbros, which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith-forming Qtz-diorites, contain Opx-bearing, trondhjemitic leucosomes surrounding Hbl + Opx + Pl mafic mesosomes forming typical agmatitic structures. Hornblende–Plagioclase equilibria, applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes, yield temperatures in the range 850–910 °C for core-to-core pairs and in the range 1000–1075 °C for rim-to rim pairs, at pressures of about 0.7 GPa. SHRIMP zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20 Ma from 465 to 485 Ma, with most ages clustered at about 470 Ma. The age of metagabbros is 473 ± 7 Ma for older zircons and 454 ± 4 for younger zircons. These ages are almost coincident within error with the age of host migmatites (477 ± 5 Ma) and those of batholith intrusion of 476 ± 9 Ma and 475 ± 3 Ma for Qtz-diorites and 475 ± 5 Ma for granites. Zircon overgrowths of these intrusive rocks yield sages clustered around 450 Ma, revealing a protracted thermal history, more complex than previously believed. The geochemical study reveals that Qtz-diorites, tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma. A weak adakitic tendency, with Sr/Y > 15 in several samples, implies the presence of Grt in the source or magma chamber at a minimum pressure of 1.0 GPa, higher than the depth of emplacement at 0.7 GPa. The high temperature of magma emplacement, which induced the incipient self-granulitization of early magmatic pulses, together with the cotectic-like fractionation linking coeval Qtz-diorites, tonalites and granodiorites, is compatible with fractionation at the lower crust of a deep-generated, infracrustal, (sublithospheric?) intermediate magma.

In the Southern Pyrenees there are Upper Carboniferous-Lower Permian sedimentary basins with a significant volume of volcanic material derived from explosive eruptions (rhyolitic ignimbrites and andesitic flows). These basins are spatially associated with granodiorites and dacitic dykes emplaced in Variscan basement rocks. U-Pb SHRIMP dating of zircons extracted from three granodiorites, an andesitic flow, a dacitic dyke and six ignimbrites, revealed that magmatism occurred over an extended period of thirty eight million years, from ca. 304. Ma to ca. 266. Ma (Upper Carboniferous-Middle Permian). A scattering of zircon ages in each sample shows that the history of melt crystallization was complex, with more than one zircon-forming event in each magma chamber. The prolonged crystallization history was transferred to the product of the eruptions. A chronological link between the deep-seated magma chambers and processes in eruptions was identified on the basis of four overlapping intervals at: ca. 309-307. Ma (Upper Carboniferous), ca. 304-296. Ma (Upper Carboniferous-Lower Permian), ca. 294-282. Ma (Lower Permian), and ca. 276. Ma (Lower Permian). The variation of zircon U/Th ratios exposes a tendency for an increase in mafic sources as crystallization advances in the Permian. Zircons probably crystallized from melt phases related to both a felsic-intermediate metaluminous source from ca. 310-293. Ma (mostly 0.1., This paper is a contribution to research projects:PLUVOLC-CGL2010/22022 (Spain), GONDWANA-PTDC/CTE-GIX/ 110426/2009 and GOLD-PTDC/GEO-GEO/2446/2012 (Portugal) and IGCP project 574: Bending and Bent Orogens, and Continental Ribbons and IGCP 597—Amalgamation and Breakup of Pangaea: the type example of the supercontinent cycle (UNESCO-IUGS).

This work shows the results of a U–Pb SHRIMP zircon geochronological study of the central part of the Gredos massif (Spanish Central System batholith). The studied batholith is composed of several granodiorite and monzogranite tabular bodies, around 1 km thick each, intruded into partially molten pelitic metasediments. Granodiorites and monzogranites, belonging to three distinct intrusive bodies, and samples of anatectic leucogranites have been selected for SHRIMP U–Pb zircon geochronology. Distinct age groups, separated by up to 20 Ma, have been distinguished in each sample. Important age differences have also been determined among the most representative age groups of the three analyzed granitoid bodies: 312.6 ± 2.8 Ma for the Circo de Gredos Bt-granodiorites (floor intrusive layer), 306.9 ± 1.5 Ma for the Barbellido-Plataforma granitoids (top intrusive layer) and 303.5 ± 2.8 Ma for Las Pozas Crd-monzogranites (middle intrusive layer). These age differences are interpreted in terms of sequential emplacement of the three intrusive bodies, contemporary with the Late Paleozoic D3 deformation phase. The anatectic leucogranites are coeval to slightly younger than the adjacent intrusive granodiorites and monzogranites (305.4 ± 1.6 Ma for Refugio del Rey leucogranites and 303 ± 2 Ma for migmatitic hornfelses). It is suggested that these anatectic magmas were generated in response to the thermal effects of granodiorite intrusions. Thermal modeling with COMSOL Multiphysics® reveals that sequential emplacement was able to keep the thermal conditions of the batholith around the temperature of zircon crystallization in granitic melts (around 750 °C) for several million of years, favoring the partial melting of host rocks and the existence of large magma chambers composed of crystal mush prone to be rejuvenated after new intrusions.

Fil: Romero, Jorge; Saltori. Universidad de Atacama; Chile Fil: Morgavi, Daniele. Università di Perugia; Italia Fil: Arzilli, Fabio. University of Manchester; Reino Unido Fil: Daga, Romina Betiana. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Fil: Caselli, Alberto Tomás. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Fil: Reckziegel, Florencia Mabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentina Fil: Viramonte, Jose German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentina Fil: Díaz Alvarado, Juan. Universidad de Atacama; Chile Fil: Polacci, Margherita. University of Manchester; Reino Unido Fil: Burton, Mike. University of Manchester; Reino Unido Fil: Perugini, Diego. Università di Perugia; Italia Fil: Daga, Romina Betiana. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Caselli, Alberto Tomás. Universidad Nacional de Río Negro. Río Negro, Argentina After 54 years since its last major eruption in 1961, Calbuco Volcano (Ensenada, Southern Chile) reawakened with few hours of warning on 22 April 2015 at 18:05 local time. The main explosive eruption consisted of two eruption pulses (lasting ~1.5 and 6 h each one) on 22 and 23 April, producing stratospheric (>15 km height) eruption columns. The erupted materials correspond to porphyritic basaltic andesite (~55 wt.% of SiO2). The tephra fall affected mainly the area northeast of the volcano and the finest ash was deposited over Southern Chile and Patagonia Argentina. We studied the tephra fall deposits of both pulses in terms of stratigraphy, distribution, volume, emplacement dynamics and eruption source parameters. Here, we show field observations that have been made 5-470 km downwind and distinguish five layers (Layers A, B, B1, C and D) representing different stages of the eruption evolution: eruption onset (Layer A; pulse 1), followed by the first paroxysmal event (Layer B; pulse 1), in some places interbedded by layer B1, tentatively representing the sedimentation of a secondary plume during the end of pulse 1. We recognized a second paroxysm (Layer C; pulse 2) followed by the waning of the eruption (Layer D; pulse 2). The total calculated bulk tephra fall deposit volume is 0.27 ± 0.007 km3 (0.11-0.13 km3 dense rock equivalent), 38% of which was erupted during the first phase and 62% during the second pulse. This eruption was a magnitude 4.45 event (VEI 4 eruption) of subPlinian type.

This study sheds light on the tectonic and structural knowledge of the mélange facies located to the south of Chañaral city, Chile. The Chañaral mélange has been related to an accretionary prism at the western active continental margin of Gondwana. Based on the fossil content, the original turbidite sequence would have been deposited during Devonian to Carboniferous times. The Chañaral mélange is included in the Las Tórtolas Formation, which corresponds to the Paleozoic metasedimentary basement located in the Coastal Range in northern Chile. It consists of a monotonous sequence of more than 90% of interbedded sandstones and shales, with a few limestones, pelagic chert, conglomerates and basic volcanic rocks, metamorphosed to the greenschist facies. In the study area, the Las Tórtolas Formation is divided into two structural domains separated by a major reverse dextral structure, called here the Infieles fault. To the east, the Las Tórtolas Formation is characterized by a brittle–ductile deformation, defined by the original sedimentary contacts in the turbiditic sequence. Besides, thrust faults and associated thrust propagation folds promotes a penetrative axial plane foliation. Mélange facies are located to the west of the Infieles fault. Although lithologies comprising this domain are similar to the rest of the Las Tórtolas Formation, mélange facies (ductile domain) are characterized by the complete disruption of the original architecture of the turbidite succession. The most significant structures in the mélange are the ubiquitous boudinage and pinch and swell structures, asymmetric objects, S–C structures and tight to isoclinal folds. This deformation is partitioned in the Chañaral mélange between linear fabric domains (L), characterized by quartzite blocks with prolate shape in a phyllite matrix with pencil structures, and linear-planar fabric domains (L-S), where quartzite objects show oblate shape and phyllites present a penetrative foliation. The intensity of deformational process is reflected in the high aspect ratios yielded by the quartzite constrictive (L) and flattened (L-S) object axes. Meso-scale shape preferred orientation (SPO) has been compared with quartz microtextures in quartzite blocks, resulting aspect ratios considerably lower than those obtained from the mesoscopic fabric. Main deformation mechanism observed in quartz microtextures are bulging-subgrain rotation recrystallization and dissolution-precipitation creep for pure and impure quartzites respectively. The temperatures deduced from these microtextures are between 350 and 400 °C, which coincides with the greenschists facies metamorphism observed in the Las Tórtolas and the mélange facies. Extremely dissociation between micro- and meso-scale deformation could be generated by dissolution at high differential stress in the boundaries of the quartzite layers and precipitation at low differential stress parts, which would increase the aspect ratio of the lenses whereas internally, quartz would have remained virtually unstrained. We propose here a tectonic setting for the Chañaral mélange formation based on the geodynamic evolution of the western active margin of South America during Late Paleozoic to Early Jurassic. Thus, the study area is located in a LT-HP zone of an accretionary complex, where rocks from the subduction channel (mélange facies) and the basal domain of the prism (brittle–ductile domain of the Las Tórtolas Formation) are in contact through the Infieles fault.

The Flamenco pluton is part of a N–S alignment of Late Triassic to Early Jurassic intrusive belt comprising the westernmost part of the Coastal Range batholith in northern Chile. The Jurassic-Cretaceous voluminous magmatism related to subduction in the western active continental margin of Gondwana is emplaced in the predominantly metasedimentary Paleozoic host-rocks of the Las Tórtolas formation, which in the northern area of the Flamenco pluton present an intense deformation, including the Chañaral mélange. Geochemically, the Flamenco pluton shows a wide compositional variability (SiO2 between 48wt % and 67wt %). Gabbros, Qtz-diorites and tonalites, mesocratic and leucocratic granodiorites are classified as calc-alkaline, calcic, magnesian and metaluminous magmatism. Flamenco granitoids define cotectic linear evolution trends, typical of magmatic fractionation processes. Geochemical trends are consistent with magmas evolved from undersaturated and low-pressure melts, even though the absence of transitional contacts between intrusive units precludes in-situ fractionation. Although some granodioritic samples show crossed geochemical trends that point to the compositional field of metasediments, and large euhedral prismatic pinnite-biotite crystals, typical Crd pseudomorph, are observed in contact magmatic facies, geochemical assimilation processes are short range, and the occurrence of host-rocks xenoliths is limited to a few meters from the pluton contact. A thermal approach to the emplacement process has been constrained through the thermobarometric results and a 2D thermo-numerical model of the contact aureole. Some Qtz-diorites and granodiorites located in the north area of the pluton exhibit granulitic textures as Hbl-Pl-Qtz triple junctions, poikiloblastic Kfs and Qtz recrystallization. The Hbl–Pl pairs have been used for the thermobarometric study of this metamorphic process, resulting granoblastic equilibrium temperatures between 770 and 790 °C, whereas Hbl–Pl pairs in domains that preserve the original igneous textures yield temperatures above 820 °C. This is characteristic of self-granulitization processes during the sequential emplacement of composite batholiths. In addition, the thermal modeling was used in order to compare the expected and observed thermal contact aureole of the intrusive body. Model P-T conditions have been established between 3 and 4 kbars (extracted from the thermobarometric results), and temperatures between 1159 °C (liquidus temperature for a tonalitic composition) and 992 °C (fixed at the rheological threshold of a 50% crystal fraction). The thermal modeling estimates a homogeneous contact aureole, where the established temperatures for the melting reactions in the host-rocks are located at distances between 200 and 650 m from the magma chamber boundary, whereas the temperatures for Crd stabilization extend 1500 m far from the contact in the case of the emplacement at liquidus temperatures and 4 kbars. According to field observations, the contact aureole presents a scarce development in the northern area of the Flamenco pluton, with few migmatite outcrops and less than 1 Km in thickness for Crd-schists. However, in the southern contact, partially melted rocks are described at distances up to 2 km from the Flamenco pluton boundary. The processes of self-granulitization and the differences between the observed and calculated (by the thermal modeling of one single pulse) contact aureole suggests a process of incremental emplacement for the Flamenco pluton, by accretion of magmatic pulses from north to south (in its current position), where the thermal maturity reached through the repeated magmatic intrusion generates a more extensive area of high-grade metamorphism.

Three granitoid bodies in the central part of the Gredos massif (Spanish Central System batholith) are tabular, around 1 km in thickness, and intruded into a migmatitic middle crust during the D3 deformation phase of the Variscan Orogeny. Petrologically, they are composed of Bt-granodiorite and Crd-monzogranite, and they show varying abundance of large (cm-scale) feldspar megacrysts. A detailed study of the shape preferred orientation (SPO) magmatic fabric defined by these megacrysts, together with a kinematic analysis of the structures due to interactions between them, and the measurement of quartz c -axis fabrics in migmatites and granitoids, suggests that granitic magma and country rocks were mechanically coupled during deformation. The emplacement took place along large-scale, extensional shear zones active during the first stages of the D3 phase. The shape of the SPO ellipsoids varies from constrictional at the centre of the granitic bodies, to flattening or even oblate at their external contacts with the migmatitic host rocks. The favoured interpretation of this spatial fabric variation is the overprinting of the emplacement fabrics by a constrictional tectonic regime associated with the growth of tabular magma chambers along extensional detachments, followed by shear zone development commonly at the top of the granitic bodies. The entire structure was later folded during the last stages of the D3 phase.

New U–Pb SHRIMP zircon ages together with a geochemical and isotopic (Sr and Nd) study have been carried out on a hornblende gabbro, a biotite granodiorite, and host andesitic rocks of the North Patagonian batholith at Paso Puyehue, close to the Argentinean-Chilean border. The results yield ages of 18 ± 1 Ma for the gabbros and 12.4 ± 0.3 Ma for the granodiorites. An age of 125 ± 2 Ma was obtained for the host volcanic rock, and an age of 22 ± 2 Ma for the basaltic trachy-andesite. The intrusive suite is composed of dominant hornblende-biotite granodiorites and subordinate hornblende gabbros and diorites. These form a typical metaluminous calc-alkaline series. The time-isotopic magmatic evolution of plutonic and volcanic rocks shows an early Eocene beginning of disparate isotopic ratios as a consequence of coeval primitive and more crustal compositions with no spatial constraints. This disparate isotopic assemblage is also coeval with the presence of the Aluk-Farallon-SAM triple junction at this latitude and the shift of the calc-alkaline magmatism from the batholith axis to the back-arc. Considerations are made for the possibility of a Paleogene slab window by slab detachment, and the development of a new subduction front since the beginning of Nazca Plate development at the 23 Ma major ocean plate rearrangement.

A petrological and geochronological study of Cordilleran granitoid intrusions in the Bariloche area (Argentina) point to a complex time-compositional evolution of magmatic processes in relation with oblique subduction of the Phoenix plate below the South America active margin during Jurassic times. The observed geochemical variations in both major and trace elements, together with the textural and mineralogical relations, point to a roughly defined, overall process of magmatic “filtering” linking all the intrusive batholithic rocks of the Bariloche area. These data suggest that the composition of the parental magma that underwent fractionation may be an intermediate magma with SiO 2 = 58–60 wt%, MgO = 2.5 wt%, FeO = 6.5 wt%, CaO = 6.1. These are coincident with the typical compositions of evolved andesites. Magnetite, amphibole and plagioclase are the main phases involved in the fractionation process. According to Hbl thermobarometry, fractionation may have taken place, at least in part, at shallow pressures ( P = 0.5–1.5 kbar), possibly at the level of emplacement. The coupled observations of the two pressure dependent ratios, namely Sr/Y and La/Yb are pointing to a low-pressure, low-temperature final fractionation dominated by only Pl. The geochronologic study by U–Pb SHRIMP zircon determinations of 14 samples from granites, tonalites and diorites yield a broad range of about 20 Ma, between 150 and 170 Ma at the Medium Jurassic. The batholith was accomplished by a protracted magmatic activity that lasted for about 20 Ma. This time is much longer than the time elapsed from intrusion to complete crystallization of shallow magma chambers. It is concluded that amalgamation of discrete magma pulses is the dominant process that built-up the batholith. The observed structures suggest that the fractures conditioning the emplacement of the magma batches were arranged en echelon and show a right-stepping. The resulting geometry is compatible with the activity of a large-scale, sinistral, N–S trending, strike-slip fracture zone permitting the emplacement of each magma pulse. This major, strike-slip fault system should be deeply entrenched in the crust to allow intruding magmas generated and fractionated at depth. Because batholith generation is a direct consequence of subduction, structural relations and ages can be used to constraint the plate motion relations during Jurassic in this region of the South America active margin.

This paper describes in detail an example of a calc-alkaline batholith, located in the Gredos massif (Spanish Central system batholith, Iberian Variscan massif), intruded into pelitic metasediments at shallow crustal levels. The igneous rocks of the study area are divided into three main groups according to their petrographic and geochemical (major, trace and rare earth element) characteristics: calc-alkalic (Qtz diorites and tonalites), transitional (Bt granodiorites), and alkali-calcic series (typically composed of Crd monzogranites). A paradoxical feature of the Crd monzogranites is the presence of euhedral Crd crystals in strong disequilibrium within a non-anatectic monzogranite. The field relations and geochemical data strongly suggest a process of bulk assimilation of the host metasediments by the intruding Bt granodiorites to generate the Crd monzogranites. Geochemical trends defined by the Crd monzogranites clearly depart from cotectic-like evolutions of liquid composition, and point to the composition of the metapelitic host-rocks. Laboratory experiments have been performed to check this hypothesis, and the results show that cordierite forms in a local domain where the reactants of the peritectic reaction Bt + Qtz + Pl + Als → Crd + Kfs + Melt are located. The process includes the isolation of metapelitic xenoliths in the granodiorite magma (a feature observed in the field), partial melting of these xenoliths with generation of cordierite as a peritectic phase in local equilibrium with the melt inside the entrapped xenolith, and the survival of these crystals because the residual melt of the granodiorite magma converges compositionally toward the peraluminous leucogranite generated in the relic xenolith. In turn, the granodiorite gains K released from the xenoliths, inducing the crystallization of K-feldspar and shifting to a Crd monzogranitic composition. Inherited zircons are present in the Crd monzogranites, similar to those found in the host metasediments; these are virtually absent in the intrusive Bt granodiorites. Mass-balance calculations and estimates based on the amount of Crd present have been applied to quantify the extent of assimilation in the petrogenesis of the granitoids; the volume of assimilated material can be high, depending on the proximity to the contacts with the xenoliths and the amount of pelitic material in the metasediments. © The Author 2010. Published by Oxford University Press. All rights reserved.

This paper gives the results of a set of laboratory experiments designed to analyse the petrological implications of mantle wedge plumes—large buoyant structures predicted by thermomechanical numerical modelling of subduction zones. A particular design of layered capsule was used to simulate the complex multilayer formed by intense flow within the mantle wedge as predicted by numerical models. A basaltic [mid-ocean ridge basalt (MORB)-derived amphibolite] component was sandwiched between two adjacent layers of a sedimentary (Bt-rich metagreywacke) component. Conditions were fixed at temperatures of 1000–1200°C at pressures of 1·5–2·0 GPa. Our results suggest that significant volumes of hybrid, Cordilleran-type granodioritic magmas can be generated by sub-lithospheric partial melting of a mechanically mixed source. Partial melting of the end-members is not buffered, forming granitic (melting of metasediment) and trondhjemitic (melting of MORB) melts in high-variance assemblages Melt + Grt + Pl and Melt + Grt + Cpx, respectively. However, the composition of melts formed from partial melting of metasediment–MORB melanges is buffered for sediment-to-MORB ratios ranging from 3:1 to 1:3, producing liquids of granodiorite to tonalite composition along a cotectic with the lower-variance phase assemblage Melt + Grt + Cpx + Pl. Our model explains the geochemical and isotopic characteristics of Cordilleran batholiths. In particular, it accounts for the observed decoupling between major element and isotopic compositions. Large variations in isotopic ratios can be inherited from a compositionally heterogeneous source; however, major element compositions are more strongly dependent on the temperature of melting rather than on the composition of the source.